To improve the optical signal transmission efficiency, in the prior art, two orthogonal polarization states of the optical signal are used to transmit two independent optical signals at the same time in the same bandwidth through polarization multiplexing, so as to double the channel transmission efficiency.
Compared with a direct detection manner, coherent technologies between the two orthogonal polarization states of the optical signal can produce an optical-signal-to-noise ratio gain about 3 dB, and can fully compensate effects of Chromatic dispersion (CD, Chromatic Dispersion) and polarization mode dispersion (PMD, Polarization Mode Dispersion) in the channel through electric equalization technologies. Over 100 Gb/s long-distance transmission may be implemented by using polarization multiplexing combined with coherent receiving technologies.
A typical polarization multiplexing coherent receiver in the prior art is specifically as shown in FIG. 1. The received optical signals are divided into x/y-path signals through a polarization beam splitter 101, and the x/y-path signals are fed into 90° frequency mixers 103x and 103y, and then digital signals Ix, Qx, Iy, and Qy with sampling of N times (N is generally 2) are obtained through a photoelectric detector 104 and an analog/digital converter 105.
Ix, Qx, Iy, and Qy are input into x/y-path dispersion compensators 106x and 106y for dispersion compensation, respectively, and a synchronizer 110 synchronizes training sequences after the completion of dispersion compensation, that is, the synchronizer 110 correlates the signals on which dispersion compensation is performed with the training sequences to obtain a correlated peak. Then, a synchronization result is obtained according to the correlated peak, and a condition of polarization cross is determined according to the synchronization result, so that a polarization compensator 107 adjusts filter coefficients.
Afterwards, the signals on which the dispersion compensation is performed are input into the polarization compensator 107 to complete polarization de-multiplexing and equalization, and then the training sequences and the equalized signals are input into phase restorers 108x and 108y respectively for phase restoration and finally input into decoders 109x and 109y, so as to restore and obtain original bit stream data.
However, in the prior art, during the synchronization of the training sequences, the signals after dispersion compensation are correlated with the training sequences to obtain the correlated peak. At this time, the x-path of signals and the y-path of signals are mixed together, so that the x-path signals and the y-path signals both have great impairment. If the x-path signals are directly correlated with the x-path training sequences, or the y-path signals are directly correlated with the y-path training sequences, a condition that a peak is unobvious or multiple peaks exist may occur, and therefore, it is easy to cause incorrect synchronization of the training sequences, thereby reducing synchronization stability of the signals.